1. Field of the Invention
The present invention relates to shift control apparatus and method for automatic transmission which are capable of controlling a shift operation of an automatic transmission (or. so-called, a shift change) in which release and engagement of a plurality of friction elements are carried out with a combination of the plurality of power transmitting friction elements (clutches) so that revolution speed and torque inputted from the engine are shifted at a shift ratio related to a target shift stage and the shifted engine speed and torque are outputted to road wheels.
2. Description of the Related Art
In a case where it becomes necessary to change a selected shift stage on the basis of a vehicular driver accelerator operation and a vehicle speed, the automatic transmission carries out a shift operation (the shift change). It is not always necessary to instantaneously carry out the shift operation. In an ordinary case, while a friction element (or a clutch) related to a presently selected shift stage is gradually released, another friction element related to a next selection scheduled target shift stage is gradually engaged. Then, both of the friction elements are temporarily slip engaged and, thereafter, the shift operation is finished. Or, a temporary neutral state is set with the friction element (or clutch) related to the presently selected shift stage released and the friction element related to a next scheduled target shift stage is engaged. Then, the shift operation is finished. This is because it is necessary to prevent a, so-called, engine racing and an abrupt variation of a road wheel driving force from occurring. However, in the automatic transmission in which the ordinary shift operation described above is carried out, the following inconveniences occur. That is to say, in the ordinary shift operation, it requires a considerable time until the shift operation is finished. Hence, even though an accelerator pedal is depressed by a driver who desires a vehicular acceleration, a vehicle cannot reach to a longitudinal acceleration direction that the driver has desired and an acceleration performance of the vehicle cannot be satisfied. Furthermore, a fuel consumption becomes worse. On the other hand, along with a development of a recent electronics control technique, a control technique as described in a Japanese Patent Application First (Tokkai) Publication No. Heisei 9-295528 published on Nov. 18, 1997 (which corresponds to a U.S. Pat. No. 5,839,989 issued on Nov. 24, 1998) has been proposed in which a quick (replacement) shift from one of the friction elements which is related to the presently selected shift stage to the other of the friction elements which is related to the next selection scheduled shift stage is carried out and each of the friction elements is controlled to finish the shift operation in a short period of time.